Hydraulically driven tractor

ABSTRACT

A large scale hydraulically driven downhole reciprocating tractor to display substantially continuous advancement. The tractor may be configured to pull coiled tubing downhole in a continuous manner so as to substantially avoid static frictional resistance to the pulling. As a result, the tractor and coiled tubing may achieve up to about twice the depth into the well otherwise attainable. Additionally, the tractor may include a fail safe mechanism whereby a plurality of pilot operated valves are employed to ensure that immobilization of at least one anchor of the tractor is present at all times during downhole advancement to thereby avoid the undesired effects of coiled tubing spring-back.

REFERENCE TO RELATED APPLICATION(S)

This Patent Document claims priority under 35 U.S.C. §119(e) to U.S. Provisional Application Ser. No. 60/883,115, entitled Flow Driven Coiled Tubing Tractor, filed on Jan. 2, 2007 which is incorporated herein by reference.

FIELD OF THE INVENTION

Embodiments described relate to tractors for pulling coiled tubing and other equipment through an underground well. In particular, embodiments of tractors are described for pulling equipment downhole in a continuous reciprocating manner.

BACKGROUND OF THE RELATED ART

Coiled tubing operations may be employed at an oil field to deliver a downhole tool to an operation site for a variety of well intervention applications such as well stimulation, the forming of perforations, or the clean-out of debris from within the well. Coiled tubing operations are particularly adept at providing access to highly deviated or tortuous wells where gravity alone fails to provide access to all regions of the wells. During a coiled tubing operation a spool of pipe (i.e., a coiled tubing) with a downhole tool at the end thereof is slowly straightened and forcibly pushed into the well. For example, a clean out tool may be delivered to a clean out site within the well in this manner to clean out sand or other undesirable debris thereat.

Unfortunately, the coiled tubing is susceptible to helical buckling as it is pushed deeper and deeper into the well. That is, depending on the degree of tortuousness and the well depth traversed, the coiled tubing will eventually buckle against the well wall and begin to take on the character of a helical spring. In such circumstances, continued downhole pushing on the coiled tubing simply lodges it more firmly into the well wall ensuring its immobilization (i.e. coiled tubing “lock-up”) and potentially damaging the coiled tubing itself. This has become a more significant matter over the years as the number of tortuous or deviated extended reach wells have become more prevalent. Thus, in order to extend the reach of the coiled tubing, a tractor may be incorporated into a downhole portion thereof for pulling the coiled tubing deeper into the well.

Unfortunately, most tractors that are currently available are somewhat incompatible with coiled tubing equipment from a powering standpoint. That is, the tractor is likely to be an electrically driven piece of equipment whereas the coiled tubing itself is hydraulic in nature. For example, the described coiled tubing is advanced into the well with some degree of fluid pressure provided thereto. At a minimum this may help to ensure that the integrity of the coiled tubing is maintained and that it does not collapse in the face of higher external pressure. For example, between a few hundred pounds per square inch (PSI) and several thousand PSI of pressure may be provided within conventionally sized coiled tubing for this purpose. Furthermore, once the downhole tool is delivered to the application site, a significantly higher pressure fluid may be pumped through the coiled tubing and ultimately to the tool.

In order to accommodate the presence of an electrically driven tractor in line with the coiled tubing, an electric cable may be provided through the coiled tubing and to the tractor. However, unless the inner diameter of the coiled tubing is undesirably increased, this reduces the available internal flow space of the coiled tubing. Alternatively, the electric cable may be provided integrally with the wall of the coiled tubing. However, in this case significant obstacles to the manufacture of the coiled tubing itself may be presented. Furthermore, regardless of the exact positioning, the addition of a separate electric cable may significantly increase the total weight of downhole equipment, given that the well depths involved are likely in the neighborhood of several thousand feet. Thus, the total achievable depth of the coiled tubing may be reduced as a result. Additionally, given the independent nature of the coiled tubing and an electric cable, other challenges may be presented in attempts to maintain compatible tensions and control simultaneously through such separate lines of equipment.

In order to avoid problems associated with the electric cable, attempts have been made to eliminate the cable by employment of a hydraulically driven tractor in place of an electric tractor. In the case of large scale tractors for pulling in the neighborhood of 5,000 lbs. or more with a tractor length of less than about 30 feet, this involves providing configurations of reciprocating tractors that may be powered with the available coiled tubing hydraulics. Reciprocating tractors are those that include at least two housings for anchoring and advancing relative to the well wall in an alternating manner. Unfortunately, however, the removal of electronic control from the tractor has left configurations of hydraulic tractors that fail to advance in a continuous manner through the well. Whether it be failure to maintain continuous advancement of the reciprocating tractor or failure to provide reliable anchoring during tractor advancement, currently available and contemplated configurations of large scale hydraulic tractors remain substantially deficient in terms of continuous downhole advancement. Thus, the total achievable coiled tubing depth attainable with current hydraulic tractors remains limited.

SUMMARY

A hydraulically driven tractor is provided for advancement within a well. The tractor includes a piston running through a first housing and a second housing and having separate piston heads disposed within each of the housings. The first head within the first housing is movably responsive to an influx of hydraulic pressure into the first housing whereas the second housing is movably responsive to this movement of the first head.

A hydraulically driven tractor may also be provided that includes an uphole assembly and a downhole assembly, each having anchors for immobilization thereof. A plurality of pilot operated valves may be coupled to each of the anchors for selectively deactivating immobilization of one of the anchors to provide lateral mobility to its associated assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side cross-sectional view of an embodiment of a hydraulically driven tractor of anchor and housing components disposed within a well.

FIG. 2 is a side cross-sectional view of the tractor of FIG. 1 with a comparative depiction of housing hydraulics therebelow.

FIG. 3 is a side cross-sectional view of the tractor of FIG. 1 with a comparative depiction of anchor hydraulics therebelow.

FIGS. 4A-4C are depictions of the tractor of FIG. 1 moving from the position of FIG. 4A to the position of FIG. 4C in a reciprocating manner.

FIG. 5 is a depiction of the tractor of FIG. 1 employed in an operation at an oil field.

FIG. 6 is a flow-chart summarizing an embodiment of employing a hydraulically driven tractor.

DETAILED DESCRIPTION

Embodiments are described with reference to certain downhole tractors for use in an oil well. In particular, dual anchor reciprocating double armed embodiments are described. However, a variety of configurations may be employed. Regardless, embodiments described may include the ability to advance downhole in a continuous manner, maintaining downward movement and substantially avoiding static friction. In fact, certain embodiments may also maintain a degree of anchoring at all times during advancement so as to avoid the occurrence of spring-back as detailed below.

Referring now to FIG. 1 an embodiment of a hydraulically driven tractor 100 is shown disposed within a production region 120 of a well 125. The tractor 100 may be directed to this location to aid in hydrocarbon recovery efforts from the production region 120. The tractor 100 has adjacent assemblies 150, 160 that each includes a housing 101, 105, an anchor 170, 180 and actuators 140, 145 therefor. A piston 110 is provided that is coupled to uphole equipment and runs through the tractor 100 including through the housings 101, 105 and anchors 170, 180 as shown. As detailed further below, the tractor 100 may be employed to pull the equipment, such as coiled tubing 525, through the well 125 and to the site of interest for performance of an application thereat (see FIG. 5).

Embodiments of the tractor 100 described herein may be particularly adept at traversing highly deviated extended reach wells which may be highly tortuous. In fact, the depicted tractor 100 may be configured for continuous advancement of the noted piston 110 in a downhole direction (to the right in the depiction of FIG. 1). This continuous downhole movement of the tractor 100 and may dramatically increase the well depth attainable thereby. For example, conventional coiled tubing that is spooled at the well surface and coupled to the piston 110 of a five thousand pound tractor 100 as described herein may be advanced in excess of five thousand feet further through a tortuous well 125 due to use of the tractor 100.

Continuing with reference to FIG. 1, the first assembly 150, referred to herein as the uphole assembly 150, includes the uphole housing 101, the uphole anchor 180, and the uphole actuator 140. As detailed further herein, the uphole anchor 180 and actuator 140 may be employed to direct immobilization, which may include centralization of the uphole assembly 150. Additionally, centralization may occur in conjunction with mobility of the assembly 150 thereby decreasing the amount of time required for immobilization. Regardless, as depicted in FIG. 1, the uphole housing 101 may be employed to direct the positioning of the uphole assembly 150 relative to the piston 110. The downhole assembly 160 similarly includes the downhole housing 105, the downhole anchor 170, and the downhole actuator 145. Again, the downhole anchor 170 and actuator 145 may be employed to direct immobilization or centralization of the downhole assembly 160 whereas the downhole housing 105 may be employed to direct the positioning of the downhole assembly 160 relative to the piston 110. As alluded to above, for the embodiments described herein, the anchors 170, 180 are deployed for centralizing when not in a state of immobilization. With such constant deployment, the time between lateral mobility and full immobilization of a given assembly 150, 160 following a pressurization switch may be significantly reduced. Nevertheless, such constant deployment is not required.

Referring now to FIGS. 1 and 2, the manner in which the assemblies 150, 160 are advanced and positioned within the well 125 are described. FIG. 2, in particular reveals a hydraulic series assembly 200 between the uphole housing 101 and the downhole housing 105. As detailed further here, the hydraulic series assembly 200 is configured such that an influx of hydraulic pressure into one of the housings 101, 105 may lead to a repositioning of the opposite housing 101, 105. As a result, a reliable reciprocating movement of the tractor 100 is achieved without interruption of the forward movement of the piston 110 or any coiled tubing or other equipment coupled thereto.

Continuing with reference to FIG. 2 the hydraulic series assembly 200 includes a downhole pressurization line 210 coupled to the downhole housing 105. For sake of description here, the downhole pressurization line 210 is presented as a high pressure line for delivering an influx of high pressure to the downhole power chamber 115 from a high pressure line 209. However, as described further herein this line 210 may not actually provide pressurization at all times.

The pressurization provided by the downhole pressurization line 210 may arrive in the form of a pressurized hydraulic oil or other conventional hydraulic fluid. For example, in one embodiment, the piston 110 of the tractor 100 is coupled uphole to coiled tubing that maintains sufficient hydraulic fluid therein to avoid collapse at a minimum. A conventional choke may be positioned in line with the coiled tubing for ultimately diverting a portion of pressurized hydraulic fluid to the downhole pressurization line 210 (or alternatively to the uphole pressurization line 215 as described below). For the examples described below, a diversion of about 2,000 PSI pressure differential into the tractor 100 relative to the well 125. However, a variety of pressurization parameters may be employed.

The piston 110 of the tractor 100 runs entirely therethrough, including through the downhole housing 105 itself. A downhole piston head 119 of the piston 110 is housed by the downhole housing 105 and serves to separate the downhole power chamber 115 from a downhole return chamber 116 of the housing 105. As indicated above, pressurized hydraulic fluid is delivered to the downhole power chamber 115 by the downhole pressurization line 210. Thus, where the downhole assembly 160 is immobilized by the downhole anchor 170 as detailed below, the application of sufficient pressure to the downhole piston head 119 may move the piston 110 in a downhole direction. Accordingly, the volume of the return chamber 116 is reduced as the volume of the power chamber 115 grows. For this period, the piston 110 moves in a downhole direction pulling, for example, coiled tubing right along with it. Of note is the fact that the arms 172 of the downhole anchor 170 may be initially immobilized with trapped hydraulic fluid of about 500 PSI, for example. However, the advancement of the piston 110, pulling up to several thousand feet of coiled tubing or other equipment, may force 15,000 PSI or more on the immobilized arms 172. Regardless, the arms 172 may be of a self gripping configuration only further anchoring the downhole assembly 160 in place. That is, the arms 172 may include a self-gripping mechanism such as responsive cams relative to the well surface as detailed in U.S. Pat. No. 6,629,568 entitled Bi-directional grip mechanism for a wide range of bore sizes, incorporated herein by reference.

As the downhole piston head 119 is forced in the downhole direction as noted above, the volume of the downhole return chamber 116 decreases. Thus, hydraulic fluid therein is forced out of the downhole housing 105 and into a downhole fluid transfer line 225. The downhole fluid transfer line 225 delivers hydraulic fluid to a switching mechanism 201 of the hydraulic series assembly 200. As depicted in FIG. 2, the switching mechanism 201 includes a switch housing 270 with a switch piston 275 disposed therein. The indicated influx of hydraulic fluid into the switch housing 270 may force the switch piston 275 in an uphole direction ultimately forcing hydraulic fluid into an uphole fluid transfer line 250 of the hydraulic series assembly 200. The uphole fluid transfer line 250 delivers hydraulic fluid to an uphole return chamber 113 of the uphole housing 101. Thus, the high pressure influx of hydraulic fluid from the downhole pressurization line 210 into the downhole power chamber 115 ultimately results in an influx of hydraulic fluid into the uphole housing 101.

The influx of hydraulic fluid into the uphole housing 101 is achieved through the uphole return chamber 113. Thus, it appears as though the hydraulic fluid would act upon an uphole piston head 117 within the uphole housing 101 in order to drive it in an uphole direction. However, as described further below, the uphole anchor 180 may act to centralize the uphole assembly 150 at this stage but does not act to force its immobilization. Thus, an increase in pressure within the uphole return chamber 113 acts to move the entire uphole assembly 150 in a downhole direction. That is, the uphole assembly 150 may require no more than between about 50 PSI and about 300 PSI of pressure for the indicated moving, whereas moving of the uphole piston head 117 and all coiled tubing or other equipment coupled thereto would likely require several thousand pounds of force. Therefore, the uphole assembly 150 is moved downhole until the downhole piston head 119 reaches the downhole end of the downhole housing 105 (see also FIG. 4B).

The anchoring and hydraulic synchronization described to this point allow for the continuous advancement of the piston 110. Thus, any equipment, such as coiled tubing that is coupled thereto may be continuously pulled in a downhole direction. This is a particular result of the series hydraulics employed. That is, hydraulic pressure is applied to one of the housings 105 which thereby employs movement of the piston 110 downhole as a corollary to the downhole advancement of the opposite housing 101. There is no measurable interruption in the advancement of the piston 110. For example, the piston 110 need not stop, wait for a housing (e.g. 101) to move and then proceed downhole. Rather, the movement of the piston 110 is continuous allowing the entire tractor 100 to avoid static friction in the coiled tubing that would be present with each restart of the piston 110 in the downhole direction. As detailed below, the advantage of this continuing movement may provide the tractor 100 with up to twice the total achievable downhole depth by taking advantage of the dynamic condition of the moving system.

As detailed above, the transfer of hydraulic pressure from one housing 101 to another 105 is achieved through multiple lines 225, 250 having a switching mechanism 201 disposed therebetween as opposed to providing a single hydraulic line between the housings 101, 105. As a result, hydraulic actuation on the switching mechanism 201 may be employed to switch the condition of high pressure from the downhole pressurization line 210 to the uphole pressurization line 215 previously served by a low pressure line 214. For example, a conventional trigger such as a spool valve may be employed that is coupled to the switch piston 275 and responsive to the head of the switch piston 275 reaching the end of its stroke at the uphole side of the switch housing 270. Upon reaching such a position the spool may be employed by conventional means to switch the pressure condition from the downhole pressurization line 210 to the uphole pressurization line 215 as indicated. Thus, with the uphole anchor 180 now immobilized at this point in time as detailed below, an influx of high pressure into the power chamber 111 of the uphole housing 101 may now drive the uphole piston head 117 in a downhole direction. This may result in an initiation of a return cycle of movement from the uphole assembly 150 to the downhole assembly 160 as described below.

As indicated above, the uphole assembly 150 is now immobilized by the uphole anchor 180 as the piston 110 is advanced downhole via pressure on the piston head 117. At this time the downhole assembly 160 may be centralized but not immobilized by the downhole anchor 170 (this is detailed further in the anchor progression description below). Similar to that described above, the advancing uphole piston head 117 forces hydraulic fluid from the return chamber 113 of the uphole housing 101 and into the uphole fluid transfer line 250, thereby sending the switch piston 275 in a downhole direction and forcing hydraulic fluid into the downhole return chamber 116. Given the non-immobilizing nature of the downhole anchor 170, the influx of pressure into the downhole return chamber 116 results in the moving of the entire downhole assembly 160 in a downhole direction (see FIG. 4C). Thus, one by one, the assemblies 150, 160 continue to reciprocate their way downhole without requiring any interruption in the downhole advancement of the piston 110 or equipment pulled thereby.

Referring now to FIG. 3, the anchoring synchronization alluded to above is detailed. That is, as evidenced by the progression above, whenever an influx of high pressure is directed to the uphole side of a piston head 117, 119 (via 210 or 215), the associated assembly 150, 160 is immobilized. In other words, whenever the downhole pressurization line 210 pressurizes the downhole power chamber 115, the downhole assembly 160 is immobilized while the uphole assembly 150 remains laterally mobile (e.g. ‘centralized’ in the embodiments shown). Similarly, following the above noted pressurization switch, whenever the uphole pressurization line 215 pressurizes the uphole power chamber 111, the uphole assembly 150 is immobilized while the downhole assembly 160 becomes laterally mobile.

Continuing with reference to FIG. 3, the explanation may again begin with reference to the downhole pressurization line 210 supplying high pressure to the downhole power chamber 115. Thus, as indicated above, the arms 172 of the downhole anchor 170 may be locked in an open position immobilizing the downhole assembly 160. Upon closer examination, a series of pilot operated valves 301, 302, 303, 304, referred to herein as “pilot valves” are shown that are responsive to pressures in the noted pressurization lines 210, 215. For the downhole anchor 170, downhole pilot valves 303, 304 are depicted. In particular, a normally closed downhole pilot valve 303 is depicted that may be opened upon exposure to a high pressure condition. However, this valve 303 is coupled to the uphole pressurization line 215 and exposed to a low pressure condition at the present time. Thus, the valve 303 remains locked in a closed position preventing the outflow of hydraulic fluid from the downhole actuator 145 into the downhole actuator line 350.

The downhole actuator piston 148 remains locked in place by the presence of the trapped hydraulic fluid, holding the downhole anchor 170 and associated arms 172 open. In this manner, the trapped hydraulic fluid may immobilize the downhole assembly 160 in combination with the self-gripping mechanism referenced above. The secondary downhole pilot valve 304, detailed further below, is of a normally open variety unless exposed to a high pressure condition. This valve 304 is indeed exposed to a high pressure condition with its coupling to the downhole pressurization line 210. Thus, the closure of the downhole actuator line 350 by these pilot valves 303, 304 is doubly certain. That is, as detailed below, closure of either valve 303, 304 ensures immobilization, whereas both valves 303, 304 may need to be open in order to allow lateral mobility of the assembly 160.

As indicated, the immobilization of the downhole assembly 160 may also indicate lateral mobility or mere centralization of the uphole assembly 150. Again, upon closer examination, the uphole anchor 180 is linked to uphole pilot valves 301, 302. In particular, a normally closed uphole pilot valve 301 is provided that may be opened upon exposure to a high pressure condition. As depicted, this valve 301 is indeed coupled to the high pressure downhole pressurization line 210, thereby allowing the valve 301 to remain open. An open secondary pilot valve 302 is also depicted that allows hydraulic overflow through the uphole actuator line 325 and ultimately to a pressurized reservoir through a line 375. Thus, the uphole actuator piston 143 is mobily responsive to radial displacement of the arms 182 as described below. Therefore, the uphole assembly 150 is laterally forced downhole in a centralized manner as detailed above.

As detailed above, the uphole pilot valves 301, 302 are noted as open for lateral mobility and centralization of the uphole assembly 150 whereas the downhole pilot valves 303, 304 are noted as closed for anchoring and immobilization of the downhole assembly 160. These are the conditions present where the downhole pressurization line 210 is high pressure in nature and the uphole pressurization line 215 is low pressure. However, as the high pressure condition moves from the downhole pressurization line 210 to the uphole pressurization line 215, the tractor 100 is susceptible to spring-back of coiled tubing in an uphole direction as the once immobilized downhole assembly 160 takes on a laterally mobile character. Thus, as described below, the secondary pilot valves 302, 304 are provided to ensure that at least one assembly 150, 160 remains immobilized at all times in order to prevent spring-back of the tractor 100 (e.g. and coiled tubing) in an uphole direction.

As depicted in FIG. 3 and described above, closure of either the uphole 325 or downhole 350 actuator lines results in immobilization of the respective anchors 180, 170. Therefore, to prevent the above noted spring-back, the tractor 100 is configured such that at least one of the anchors 180, 170 is immobilized at all times. In fact, in embodiments depicted herein, the tractor 100 is configured such that both anchors 180, 170 are immobilized for a brief period as the high pressure condition switches from the downhole pressurization line 210 to the uphole pressurization line 215 and vice versa as described above. This eliminates the possibility of one of the anchors 170, 180 moving to a state of lateral mobility while the other is yet to be fully immobilized. The resulting spring-back avoidance, as it is termed here, is a feature made possible by the use of the secondary pilot valves 302, 304 noted above.

Each primary valve 301, 303 is paired with a secondary valve 302, 304 as shown in FIG. 3. All of these valves 301, 302, 303, 304 may be independently responsive to pre-determined high or low pressures so as to ensure a delay between the opening of a primary valve (e.g. 303) and its corresponding secondary valve (e.g. 304). This delay may be configured to ensure the immobilization of the anchor opposite the valve pair, such as the uphole anchor 180 opposite the downhole pilot valve pair 303, 304, prior to allowing the lateral mobility of the corresponding anchor (e.g. the downhole anchor 170). An example of this delay is described in greater detail below with reference to a switch in the downhole pressurization line 210 from a high pressure condition to a low pressure condition.

As described above, a high pressure condition in the downhole pressurization line 210 corresponds to the immobilization of the downhole anchor 170 by way of the closing off of the downhole actuator line 350 via the downhole primary 303 and secondary 304 pilot valves. However, as described above, the downhole primary pilot valve 303 is normally closed unless a condition of high pressure is introduced. Therefore, as the high pressure condition begins to switch from the downhole pressurization line 210 to the uphole pressurization line 215 that is coupled to the downhole primary pilot valve 303, this valve 303 may be opened (and valve 302 closed). At this time, perhaps prior to the immobilization of the uphole anchor 180, the opening of this valve 303 would result in opening of the downhole actuator line 350 and lateral mobility of the downhole anchor 170 but for the presence of the secondary downhole pilot valve 304. However, the closed secondary downhole pilot valves 304, 301 also responsive to the switch of the pressure condition in the line 210 may be employed to ensure a delay in the complete opening of the downhole actuator line 350 until after the closure of the line 325.

Continuing now with reference to FIG. 3, an overflow line 375 is shown where hydraulic fluid may be diverted to a pressure sub or other storage or release means. In this manner, the pressures in the valves 301, 302, 303, 304 may be kept at a level conducive to provide sufficient centralization. For example, in one embodiment a pressure sub may be employed to ensure that no more than about 500 PSI of pressure may be directed through the valves 301, 302, 303, 304 toward the assemblies 150, 160. An additional fail safe mechanism may also be provided to ensure that no pressure is trapped by the valves 301, 302, 303, 304 when the tractor 100 is turned off. Furthermore, in one embodiment the overflow line 375 is configured to allow excess hydraulic fluid to be diverted from an open valve pair (e.g. 301 and 302) in response to a centralizing anchor (e.g. 180) reducing in profile as it encounters narrowing well features during movement downhole. Similarly, this same hydraulic fluid may be returned to the centralizing anchor 180 from the overflow line 375 as the well increases in profile.

Referring now to FIGS. 4A-4C, the uninterrupted synchronization of anchoring and downhole reciprocating advancement of the tractor 100 is depicted. Starting with FIG. 4A, the tractor 100 is shown with the uphole assembly 150 distanced from the downhole assembly 160 within a production region 120 of a well 125. Much of the switch mechanism 201, anchor valves 301-304, and other features of FIGS. 2 and 3 above may be housed within a hydraulic housing of the tractor 100.

As shown in FIG. 4A, the downhole actuator 145 is locked as described above such that the downhole anchor 170 is immobilized. Thus, pressure applied to the downhole power chamber 115 and on the downhole piston head 119 advances the piston 110 downhole (see FIG. 4B). At this same time, the uphole anchor 180 may be centralizing in nature and allowing for lateral mobility of the uphole assembly 150 as depicted below with reference to FIG. 4B.

Referring now to FIG. 4B, the noted lateral mobility of the uphole assembly 150 may be effectuated by the influx of pressure into the uphole return chamber 113. That is, given the minimal amount of force required to move the assembly 150, perhaps no more than about 300 PSI of pressure, a downhole movement thereof may be seen with reference to arrow 450. Of note is the fact that it is the downhole movement of the downhole piston head 119 that has lead to the influx of pressure into the chamber 113 thereby providing the downhole movement of the uphole assembly 150. Furthermore, while the uphole piston head 117 appears to move uphole, it is actually the uphole housing 101 thereabout that has moved downhole as indicated. Indeed, the entire piston 110 continues its downhole advancement without interruption as noted below with reference to FIG. 4C.

As shown in FIG. 4C, the uphole piston head 117 appears to resume downhole advancement relative to the uphole hosing 101. However, as indicated above, the entire piston 110, including the uphole piston head 117 actually maintains uninterrupted downhole advancement. For example, once the switch piston 201 finishes its travel in the uphole direction, the above described switch in pressure conditions occurs that leads to an influx of pressure into the uphole power chamber 111. At this same time, the uphole anchor 180 is immobilized by the locking of the uphole actuator 140 as detailed above. Therefore, the uphole piston head 117 is driven to continue the downhole advancement of the entire piston 110. Indeed, this downhole advancement of the uphole piston head 117 relative to the uphole housing 101 leads to an influx of pressure into the downhole return chamber 116. Thus, with the move to a mobile state of centralization of the downhole anchor 170 at this time, as detailed above, the downhole assembly 160 advances further downhole (see arrow 475).

As indicated, embodiments described herein allow for continuous downhole advancement of the piston 110. Thus, the load pulled by the piston 110, such as several thousand feet of coiled tubing or other equipment may be pulled while substantially avoiding resistance in the form of static friction. Downhole advancement of the load is not interrupted by any need to reset or reposition tractor anchors 170, 180 or assemblies 150, 160. Thus, in the face of dynamic friction alone, the tractor 100 may be able to pull a load of up to about twice the distance as compared to a tractor that must overcome repeated occurrences of static friction. For example, where just under a 5,000 lb. pull is required to advance a load downhole, a 5,000 lb. capacity tractor of interrupted downhole advancement must pull about 5,000 lbs. after each interruption in advancement. Thus, as soon as the pull requirement increases to beyond 5,000 lbs. based on depth achieved, the tractor 100 may be able to pull the load no further. However, for embodiments of the tractor 100 depicted herein, even those subjected to a 5,000 lb. pull requirement at the outset of downhole advancement, the degree of pull requirement soon diminishes (e.g. to as low as about 2,500 lbs.). Only once the depth of advancement increases the pull requirement by another 2,500 lbs. does the 5,000 lb. capacity tractor 100 reach its downhole limit. For this reason, embodiments of tractors 100 described herein have up to about twice the downhole pull capacity of a comparable tractor of interrupted downhole advancement.

Referring now to FIG. 5, an embodiment of the hydraulically driven tractor 100 detailed hereinabove is depicted in use at an oil field 500. The tractor 100 is coupled to the downhole portion of coiled tubing 525 that is driven into a well 125 by an injector 530 at the surface of the oil field 500. A coiled tubing reel 510 supplies the coiled tubing 525 to the injector 530 as shown. The linear maintenance of the coiled tubing 525 beyond the lock-up depth is achieved by the pull thereon from the hydraulically driven tractor 100 shown.

Continuing with reference to FIG. 5, the tractor 100 is also able to deliver a tool, such as the depicted clean out tool 575 to a clean out site 580 within the production region 120. The clean out tool 575 may be delivered for the purpose of removal of debris 560 as shown in order to improve recovery of hydrocarbons from the production region 120. However, the tractor 100 may be employed to deliver a variety of other tools for any number of purposes. Regardless, as detailed above, the continuous downhole movement of the tractor 100 may allow it to deliver the tool up to about twice the depth of that achievable by a conventional reciprocating tractor and without need to overcome any intermittent spring-back or retraction uphole thereof.

Referring now to FIG. 6, a flow-chart summarizing an embodiment of employing a hydraulically driven tractor such as that detailed above is depicted. With added reference to FIGS. 4A-4C, the flow-chart details the manner of uninterrupted downhole advancement attainable by embodiments of hydraulically driven tractors 100 detailed herein. Such a tractor 100 may be employed to pull coiled tubing deeper within a well that is also pushed from the surface as noted with reference to FIG. 5.

As indicated at 610, a piston of the tractor may be coupled to the coiled tubing and positioned within the well. Once the tractor is centralized therein as noted at 615, a first housing of the tractor may initially be immobilized as indicated at 620. Thus, pressurization of a first piston head within the first housing may result in driving of the entire piston downhole (see 630). Therefore, the laterally mobile second housing may be moved in response to the pressurization as indicated at 650. Of particular note here is the fact that lateral mobility or centralization may be provided to the second housing only upon the complete immobilization of the first housing as detailed above with reference to FIG. 3. In this manner the possibility of an uphole spring-back of the tractor and coiled tubing may be avoided to ensure the efficient and continuous downhole movement thereof.

Once the second housing has moved, it may be immobilized as indicated at 660. Lateral mobility may then be provided to the first housing as indicated at 665. Further, in order to continue the driving of the piston downhole, pressurization may be switched to a second piston head with the second housing (see 670). Again, this pressurization may lead to downhole movement of the first housing as indicated at 690. Thus, in all cases, the moving of a housing of the tractor is in response to the driving of the piston downhole. Therefore, there is no requirement that the piston's downhole progression be interrupted for downhole moving of a tractor housing.

Embodiments of the tractor 100 described herein avoid the requirement of a separate electric cable for powering purposes. Nevertheless, hydraulic components which supply power to the tractor embodiments are configured such that downhole advancement of the tractor 100 is achieved in a continuous manner that avoids compromise to the efficiency of advancement or the total well depths achievable by the tractor 100.

The preceding description has been presented with reference to presently preferred embodiments. Persons skilled in the art and technology to which these embodiments pertain will appreciate that alterations and changes in the described structures and methods of operation may be practiced without meaningfully departing from the principle, and scope of these embodiments. For example, embodiments depicted herein reveal a two arm configuration for each anchor similar to that of U.S. App. Ser. No. 60/890,577. However, other configurations with other numbers of arms for each anchor may be employed. Furthermore, the foregoing description should not be read as pertaining only to the precise structures described and shown in the accompanying drawings, but rather should be read as consistent with and as support for the following claims, which are to have their fullest and fairest scope. 

1. A hydraulically driven tractor for advancement within a well, the tractor comprising: a piston; a first housing about a first head of said piston, the first head for moving responsively to an influx of hydraulic pressure into said first housing; and a second housing about a second head of said piston, the second housing to display movable responsiveness to the moving relative to the piston.
 2. The hydraulically driven tractor of claim 1 wherein said piston is coupled to a downhole conveyance line extending from a surface at an origin of the well, the moving to pull the downhole conveyance line downhole.
 3. The hydraulically driven tractor of claim 2 wherein the downhole conveyance line is coiled tubing.
 4. The hydraulically driven tractor of claim 1 further comprising a hydraulic series assembly through which said first housing is fluidly coupled to said second housing.
 5. The hydraulically driven tractor of claim 4 wherein said hydraulic series assembly comprises: a first transfer line coupled to said first housing for receiving an output of hydraulic pressure from the moving; a second transfer line coupled to said second housing to deliver the output of hydraulic pressure thereto for the responsiveness; and a switching mechanism coupled to said first transfer line and said second transfer line for transferring the output of hydraulic pressure from said first transfer line to said second transfer line.
 6. The hydraulically driven tractor of claim 5 further comprising: a first pressurization line to provide the influx of hydraulic pressure into said first housing for a first period; and a second pressurization line to provide an influx of hydraulic pressure into said second housing for a second period exclusive of said first period, said switching mechanism to regulate the periods.
 7. The hydraulically driven tractor of claim 1 further comprising: a first anchor coupled to said first housing for immobilization thereof during the moving; and a second anchor coupled to said second housing to allow lateral mobility thereof for the responsiveness.
 8. The hydraulically driven tractor of claim 7 wherein the lateral mobility of said second housing includes centralization of said second housing.
 9. The hydraulically driven tractor of claim 7 wherein said second anchor is coupled to said second housing for periodic immobilization thereof.
 10. The hydraulically driven tractor of claim 9 further comprising a primary pilot valve and a secondary pilot valve coupled to said second anchor, said second anchor to allow the lateral mobility for the responsiveness exclusively upon opening of both said primary pilot valve and said secondary pilot valve.
 11. A hydraulically driven tractor comprising: a first assembly with a first anchor for immobilization thereof, a second assembly with a second anchor for immobilization thereof, and a mechanism coupled to each of the anchors for selectively activating immobilization of the first anchor prior to deactivating immobilization of the second anchor to provide lateral mobility thereto.
 12. The hydraulically driven tractor of claim 11 wherein said mechanism comprises a plurality of pilot valves.
 13. The hydraulically driven tractor of claim 12 wherein said plurality of pilot valves comprises primary and secondary pilot valves coupled to the second anchor, the deactivating upon opening of both said primary and secondary pilot valves.
 14. The hydraulically driven tractor of claim 12 wherein the activating is achieved upon closure of one pilot valve of the plurality of point valves.
 15. The hydraulically driven tractor of claim 11 wherein the deactivating of the second anchor is for a first period exclusive of a second period of deactivating of the first anchor.
 16. The hydraulically driven tractor of claim 11 further comprising: a piston; a first housing of the first assembly to accommodate a first head of said piston, the first head for moving responsively to an influx of hydraulic pressure into said first housing during immobilization thereof, and a second housing of the second assembly to accommodate a second head of said piston, said second housing to display movable responsiveness to the moving relative to the piston during deactivating of the second anchor.
 17. A method of pulling coiled tubing downhole in a well, the method comprising: securing the coiled tubing to a downhole tractor; immobilizing a first housing of the tractor within the well; pressurizing a piston head in the first housing for driving the piston downhole; providing lateral mobility to a second housing of the tractor; and moving the second housing downhole relative to the piston in response to the pressurizing.
 18. The method of claim 17 wherein said providing further comprises centralizing the second housing.
 19. The method of claim 17 wherein the piston head is a first piston head, the method further comprising: immobilizing the second housing within the well; and switching said pressurizing to a second piston head within the second housing to continue the driving.
 20. The method of claim 19 further comprising: deactivating said immobilizing of the first housing; and moving the first housing downhole in response to said switching.
 21. The method of claim 20 wherein said deactivating further comprises centralizing the first housing.
 22. The method of claim 20 further comprising employing a plurality of pilot valves to ensure said immobilizing of the second housing occurs prior to said deactivating.
 23. A hydraulically driven tractor for advancement of a downhole conveyance line within a well, the tractor comprising: a piston coupled to the downhole conveyance line and having a first head and a second head; a first housing about the first head of the piston, wherein the first head divides the first housing into a power chamber and a return chamber, and wherein the first head moves in response to an influx of hydraulic pressure into the power chamber of the housing; and a second housing about a second head of said piston, wherein the second head divides the second housing into a power chamber and a return chamber, and wherein the second housing is fluidly coupled in series to the first housing, such that said influx of hydraulic pressure into the power chamber of the first housing causes fluid to be expelled from the return chamber of the first housing and into the return chamber of the second housing to move the second housing relative to the piston.
 24. The hydraulically driven tractor of claim 23 wherein the downhole conveyance line comprises coiled tubing.
 25. The hydraulically driven tractor of claim 23 further comprising: a first anchor coupled to the first housing for immobilizing the first housing as the first head moves; and a second anchor coupled to the second housing, but allowing lateral mobility of the second housing as the second housing moves relative to the piston. 